The present invention relates to an injection system for intermittent fuel supply in combustion chambers of an I.C. engine, and more particularly, to an accumulator-type injection system in which a nozzle needle is arranged in the injector body to be displaceable in a longitudinal direction of the injector and, in injection pauses, is configured to be held by a spring on a seat thereof in the injector body. When radial surfaces of the nozzle needle are acted upon by accumulated pressure, the needle lifts from the seat thereof in the injector body and opens up orifices. A control valve controls the injection between two switching positions whereby, in one of the switching positions, an injection passage is opened up to connect a fuel duct in the injector body with an injection pipe and, in the other of the switching positions, closes off the passage in the injection pauses.
For reducing smoke oxides and nitrogen oxides in the exhaust gas of a diesel internal-combustion engine, it is a known technique to inject water into the cylinders together with the fuel.
Gerhard Lehner in "Diesel Injection for Large Engines" in MTZ Motortechnische Zeitschrift 55 (1994) 9, pages 502, 511 and 512, describes an accumulator-type injection system. The injector used there is constructed with a spring-loaded nozzle needle. For the injection, a passage is opened up by a magnetic valve and connects the pressure accumulator with a fuel duct in the nozzle body leading to the orifices. The fuel pressure acting on the radial surfaces of the nozzle needle lifts the nozzle needle from its seat in the injector body, whereby the orifices are opened up. In the injection pauses, the nozzle needle is not loaded by the accumulated pressure but by the stationary pressure which depends on the stiffness of the spring which pushes the nozzle needle against its seat in the injector body. Because the stiffness of the spring must be high for correct functioning, the stationary pressure in the injection pauses will also not be low.
German Patent Application P 43 41 739, which is no prior publication, discloses an accumulator-type injection system whose nozzle needle is controlled hydraulically. The high pressure of the fuel accumulator acts in a control space with a piston which is connected with the nozzle needle and holds the nozzle needle in a closed position in the injection pauses. At the same time, in the injection pauses, an annulus on the nozzle needle, which is connected with the orifices, is connected by way of another electromagnetic valve with a return flow pipe. For this reason, the fuel pressure is very low in front of the orifices, between the injections, and water can be transported at a relatively low pressure in front of the orifices. For triggering the injection, the above-mentioned magnetic valves must each be switched over. The control space is relieved from pressure, and the annulus in front of the orifices is connected with the fuel accumulator.
An object of the present invention is, in an accumulator-type injection system, to provide a simple way of lowering the fuel pressure existing in front of the orifices during the injection pauses to approximately the ambient pressure.
The foregoing object has been achieved in accordance with the present invention by providing that in the injection pauses, the fuel duct is connected with the return flow pipe by a passage opened up by the control valve, and, in the injection pauses, the pump delivers water in front of the orifices via a pipe.
For controlling the injection and for lowering the fuel pressure to values below the stationary pressure corresponding to the spring stiffness, a single control valve will be sufficient for two switching positions. In a first switching position, in a known manner, an annulus on the nozzle needle situated in front of the orifices is connected by a fuel duct in the injector body with an injection pipe connected with the fuel accumulator. For this purpose, a corresponding first passage is opened up. In the second switching position, when the above-mentioned passage is closed, a second passage is opened up instead which connects the fuel duct, which leads in front of the orifices, with a return flow pipe.